How to solve the second problem on page 190 of the seventh grade mathematics volume I of Beijing Normal University Edition

How to solve the second problem on page 190 of the seventh grade mathematics volume I of Beijing Normal University Edition

Suppose there are x Chinese books and (90-x) mathematics books
0.8·(90—x)+1.2x=88
72—0.8x+1.2x=88
0.4x=16
x=40
90-40 = 50
Answer by yourself